The following references are believed to represent the state of the art:
US Published Patent Application 2004/0003399 of Cooper;
US Published Patent Application 2005/0097603 of Kikinis;
US Published Patent Application 2006/0020989 of Brun;
U.S. Pat. No. 5,523,796 to Marshall, et al.;
U.S. Pat. No. 5,623,613 to Rowe, et al.;
U.S. Pat. No. 5,880,722 to Brewer, et al.;
U.S. Pat. No. 5,933,140 to Strahorn, et al.;
U.S. Pat. No. 6,239,794 to Yuen, et al.;
U.S. Pat. No. 6,240,555 to Shoff, et al.;
U.S. Pat. No. 7,096,428 to Foote, et al.;
European Published Patent Application EP 1 024 661 of Hughes Electronics Corporation;
PCT Published Patent Application WO 97/49242 of Starsight Telecast Incorporated;
PCT Published Patent Application WO 2004/072935 of NDS Limited;
PCT Published Patent Application WO 2007/000559 of NDS Limited; and
PCT Published Patent Application WO 2007/015047 of NDS Limited.
The disclosures of all references mentioned above and throughout the present specification, as well as the disclosures of all references mentioned in those references, are hereby incorporated herein by reference.